The present invention relates to electronic communication, and in particular to the propagation of electrical signals by means of a user""s body through electrostatic coupling through a person.
Traditional forms of short-range electronic communication typically involve radiated energy. For example, radio-frequency identification (RFID) devices have been employed for some time to remotely sense parameters of interest in people or objects. An RFID device receives a wireless signal from an externally located xe2x80x9creader,xe2x80x9d which determines the parameter of interest based on the response of the RFID device to the transmitted signal. A simple application of this technology is security: an individual wears an RFID xe2x80x9ctagxe2x80x9d or badge, and a controlled-entry system unlocks a door only if the wearer""s tag is recognized as s/he approaches.
Magnetostatic RFID systems respond to magnetic fields and therefore require induction coils, which add weight and cost to the device in addition to establishing a minimum size. As explained in copending application Ser. No. 08/965,465, filed on Nov. 6, 1997, electrostatic devices offer practical advantages over magnetostatic systems since the induction coil is eliminated and electrodes can be conveniently and inexpensively deposited on substrates of widely varying shapes and materials. The ease of focusing an electric field as compared with a magnetic field also favors electrostatic systems.
In accordance with the approach disclosed in the ""465 application, a transmitter and receiver are capacitively coupled through a user and room ground. In particular, a transmitter produces low-frequency (generally 100 kHz to 10 MHz) AC signals that pass, through capacitive coupling, as displacement currents into and from the body of the user, carrying both power and data to a receiver, which is also coupled to the user""s body. Since the transmitter and receiver do not couple with one another directly, the shared room ground provides the return path for the current.
An important aspect of operation is the asymmetric arrangement of electrodes with respect to the user. An inner electrode of the receiver or transmitter is closely coupled capacitively to the user""s body such that the xe2x80x9cquasi-electrostaticxe2x80x9d field resulting from the electrode potential causes a displacement current to pass to user""s body. The outer electrode is oriented so that its coupling to the room ground is stronger than that of the inner electrode. This enforced asymmetric coupling ensures that a potential difference exists across the electrodes, thereby completing the circuit.
While highly useful in many applications, the approach of the ""465 application does not offer selectivity of communication: whenever a receiver is coupled to a transmitter by mutual contact with the user, they will necessarily intercommunicate. This approach, therefore, does not lend itself well to facilitating selection of one transmitter from among a plurality.
The present invention utilizes intrabody transmission of displacement current and RFID technology to provide physical structures that are responsive to touch; in particular, touching a selected structure conveys information, e.g., the identity of the structure and/or some associated data. In its simplest form, the structure comprises an electrical load and a pair of electrodes connected thereto. The electrodes are spaced apart (by air or other dielectric medium). A signal generator, which acts as a xe2x80x9creader,xe2x80x9d is connected to a larger electrode that may be embedded in an environmental surface. When the structure is placed in proximity to the reader electrode, application of the signal results in capacitive coupling between the reader and the electrodes of the structure. The structure is not detected by the reader, however, because the capacitive coupling is comparable for both electrodes, so the load is subjected only to a small electrical gradient. But if one of the electrodes is grounded, this symmetry is broken and current is drawn through the load. In general, grounding occurs when a person, coupled (even weakly) to environmental groundxe2x80x94e.g., by virtue of standing on the floorxe2x80x94touches one of the electrodes. The resulting loading is detected by the signal generator, confirming, for example, that the structure had been touched (or nearly touched). In some embodiments, the system may contain numerous such structures in proximity to the reader electrode, each structure having a different load; by virtue of the loading it experiences, the reader can determine which structure has been selected.
In more elaborate embodiments, the load is an RFID chip capable of imparting information to the signal generator in the form of loading variations. In the time domain, the temporal pattern of these variations can encode a sequence of bits.
Numerous applications are possible depending on where the structure and the reader are located. The structure may, for example, be embedded in a portable object, such as a package, or in the token of a game. Touching a particular object may connote selection of that item by the user, who may thereupon receive information about the item or advance the play of the game. The reader may be associated with a game board (the electrode disposed, for example, within or underneath the board), as part of an item of furniture (e.g., disposed below a table, desktop, or shelf, or behind a whiteboard), or within an architectural surface or fixture. The structure may also act as an interface, contact with the structure causing action to be taken by an associated computer. For example, the structure may specify the address of a web page; in this way, the user may select from among numerous web pages by touching a physical object associated therewith.
In accordance with the invention, wireless communication of information is accomplished using a tag unit comprising an electrical load and a pair of parallel, spaced-apart electrodes. A signal source is capacitively coupled to the electrodes without creating a significant potential difference therebetween. The signal source is configured to detect the load and derive information therefrom when current is drawn therethrough as a result of selective coupling of one of the electrodes to ground, thereby creating a potential difference across the electrodes. This approach is distinct from intrabody and interbody electrostatic communication systems such as that disclosed in the ""465 application in that coupling is inherently symmetric rather than asymmetric, so that a circuit activating the load is not ordinarily completed; it requires the intervention of a user to actively break the symmetry in order to trigger a response.